A mixed ionic conductor has a long history, and so far various types of mixed ionic conductors have been developed. In particular, an oxide ionic conductor of a solid oxide (e.g., zirconia or cerium oxide) has been used for an electrochemical device such as a fuel cell or gas sensor. As a proton conductor, e.g., SrCe1-xMxO3, CaZr1-xMxO3, or SrZr1-xMxO3 (M is a trivalent element and 0<x<1, the same is true for the following compositions unless otherwise noted) has been known. Moreover, BaCe1-xMxO3, which is an oxide of barium and cerium, has been reported as a mixed ionic conductor that conducts oxide ions and protons simultaneously. In particular, when M is Gd and x is 0.16 to 0.23, the mixed ionic conductor has higher conductivity (see JP 5(1993)-28820 A (Patent Document 1)).
Although many mixed ionic conductors have been found, only a few of them are put to practical use. At present, zirconia is used as an oxygen sensor, and SrCe1-xMxO3 or CaZr1-xMxO3 is used to detect the concentration of hydrogen in a smelting furnace. However, these mixed ionic conductors work in a limited environment and are not fully reliable. For example, when the mixed ionic conductors are boiled in water, they are decomposed over about 1 to 100 hours. Moreover, a change that is attributed to the decomposition of materials is observed at 85° C. and 85% RH (relative humidity). A conventionally known proton conductor of a perovskite oxide hardly can exist stably in high humidity. JP 2000-302550 A (Patent Document 2) discloses a mixed ionic conductor, particularly a perovskite oxide in view of stability in boiling water, such as BaZr1-x-yCexMyO3 or BaZr1-x-yCexMyO3 (0.01≦y≦0.3). However, the ionic conductivity is lower than BaCe1-xMxO3.
JP 9(1997)-295866 A (Patent Document 3) discloses Ba1-xLxCe1-yMyO3-a (L is Mg, Ca or Sr and M is La, Pr, Nd, Pm, Sm or Eu). JP 2001-307546 A (Patent Document 4) discloses BaZr1-xCexO3-p. JP 6(1994)-231611 A (Patent Document 5) discloses BaCe1-xMxO3-y. All of these crystals have the disadvantage of low conductivity at high temperatures.
The mixed ionic conductor of a solid oxide is used generally at high temperatures, and therefore is required to resist thermal shock. However, the conventional mixed ionic conductor of a perovskite oxide does not have sufficient mechanical strength and is susceptible to cracks caused by thermal shock.
Patent Document 1: JP 5(1993)-28820 A
Patent Document 2: JP 2000-302550 A
Patent Document 3: JP 9(1997)-295866 A
Patent Document 4: JP 2001-307546 A
Patent Document 5: JP 6(1994)-231611 A
As described above, a reliable mixed ionic conductor, particularly a perovskite oxide proton conductor is rare. With the progress in development of an electrochemical device such as a fuel cell, there has been a growing demand for a mixed ionic conductor that can provide high conductivity and high reliability for use under harsh environments.